An electrocardiogram (ECG) is used to measure the heart's electrical conduction system. The ECG measures electrical impulses generated by the polarization and depolarization of cardiac tissue, and translates the electrical impulses into a wave, which is used to measure the rate and regularity of heartbeats. The wave comprises a P wave, a QRS complex, a T wave, and a U wave. The QRS complex includes a plurality of R waves.
A RR interval, namely the interval between two adjacent R waves, can be used to measure a heart rate. The heart rate can vary due to physical exercise, mood, and so on.
Usually, the ECG can only be read by experts, for example medical staffs.